The present invention relates to a high temperature, corrosion and wear resistant iron-based alloy, and particularly to an alloy for use in valve seat inserts.
More restrictive exhaust emissions laws for diesel engines have driven changes in engine design including the need for high-pressure electronic fuel injection systems. Engines built according to the new designs use higher combustion pressures, higher operating temperatures and less lubrication than previous designs. Components of the new designs, including valve seat inserts (VSI), have experienced significantly higher wear rates. Exhaust valve seat inserts and valves, for example, must be able to withstand a high number of valve impact events and combustion events with minimal wear (e.g., abrasive, adhesive and corrosive wear). This has motivated a shift in materials selection toward materials that offer improved wear resistance relative to the valve seat insert materials that have traditionally been used by the diesel industry.
Another emerging trend in diesel engine development is the use of EGR (exhaust gas recirculation). With EGR, exhaust gas is routed back into the intake air stream to reduce nitric oxide (NOx) content in exhaust emissions. The use of EGR in diesel engines can raise the operating temperatures of valve seat inserts. Accordingly, there is a need for lower cost exhaust valve seat inserts having good hot hardness for use in diesel engines using EGR.
Also, because exhaust gas contains compounds of nitrogen, sulfur, chlorine, and other elements that potentially can form acids, the need for improved corrosion resistance for alloys used in exhaust valve seat insert applications is increased for diesel engines using EGR. Acid can attack valve seat inserts and valves leading to premature engine failure. Earlier attempts to achieve improved corrosion resistance were pursued through the use of martensitic stainless steels. Though these steels provide good corrosion resistance, conventional martensitic stainless steels do not have adequate wear resistance and hot hardness to meet the requirements for valve seat inserts in modern diesel engines.
Cobalt-based valve seat insert alloys are known for their high temperature wear resistance and compressive strength. A major disadvantage of cobalt-based alloys, however, is their relatively high cost. Iron-based VSI materials, on the other hand, typically exhibit a degradation in matrix strength and hardness with increasing temperature, which can result in accelerated wear and/or deformation. Iron-based alloys for use in internal combustion engine valve seats are disclosed in U.S. Pat. Nos. 5,674,449; 4,035,159 and 2,064,155.
Iron-based alloy compositions are disclosed in U.S. Pat. Nos. 6,340,377; 6,214,080; 6,200,688; 6,138,351; 5,949,003; 5,859,376; 5,784,681; 5,462,573; 5,312,475; 4,724,000; 4,546,737; 4,116,684; 2,147,122 and in Japanese Patent Nos. 58-058,254; 57-073,172 and 9-209,095.
There is a need in the art for improved iron-based alloys for valve seat inserts that exhibit adequate hot hardness, high temperature strength and low cost, as well as corrosion and wear resistance suitable for use in exhaust valve insert applications in diesel engines using EGR.
An iron-based alloy with improved corrosion resistance, hot hardness and/or wear resistance. The alloy is suitable for use in exhaust valve seat insert applications, such as diesel engines using EGR.
According to an embodiment, the iron-based alloy comprises, in weight percent, boron from about 0.005 to about 0.5%; carbon from about 1.2 to 1.8%; vanadium from about 0.7 to 1.5%; chromium from about 7 to 11%; niobium from about 1 to 3.5%; molybdenum from about 6 to 11%, and the balance including iron and incidental impurities.
According to another embodiment, a cast, iron-based tungsten-free alloy comprises, in weight percent, boron from about 0.1 to 0.3%; carbon from about 1.4 to 1.8%; silicon from about 0.7 to 1.3%, vanadium from about 0.8 to 1.5%; chromium from about 9 to 11%; manganese from about 0.2 to 0.7%; cobalt from about 0 to 4%; nickel from about 0 to 2%; niobium from about 1 to 2.5%; molybdenum from about 8 to 10%, and the balance including iron and incidental impurities. If desired, copper can be substituted partially or completely for cobalt.
According to a further embodiment, the alloy comprises about 0.005 to 0.5% boron; about 1.2 to 1.8% carbon; about 0.7 to 1.5% vanadium; about 7 to 11% chromium; about 6 to 11% molybdenum; at least one element selected from the group consisting of titanium, zirconium, niobium, hafnium and tantalum, represented by Ti, Zr, Nb, Hf and Ta, respectively, and the balance including iron and incidental impurities, such that 1% less than (Ti+Zr+Nb+Hf+Ta) less than 3.5%.
According to a preferred embodiment, the alloy is tungsten-free and includes, in weight percent, up to 1.6% Si and/or up to about 2% Mn. Preferably, the alloy can include between about 0.1 to 0.3% boron; about 1.4 to 1.8% carbon; about 0.8 to 1.5% vanadium; about 9 to 11% chromium; about 1 to 2.5% niobium; up to about 4% cobalt, more preferably about 1.5 to 2.5% cobalt; up to about 2% nickel, more preferably about 0.7 to 1.2% nickel and/or about 8 to 10% molybdenum. According to a preferred embodiment, the content, in weight percent, of boron, vanadium and niobium satisfy the condition 1.9% less than (B+V+Nb) less than 4.3% wherein B, V and Nb represent the weight % content of boron, vanadium and niobium, respectively.
Preferably, the alloy is in a hardened and tempered condition and the alloy has a martensitic microstructure including primary and secondary carbides. Preferably, the primary carbides in the alloy have a width smaller than about 10 microns, more preferably smaller than about 5 microns, and the secondary carbides in the alloy are smaller than about 1 micron. The alloy is preferably in the form of a casting. The hardened and tempered alloy preferably exhibits a hardness of at least about 42 Rockwell C. At a temperature of 800xc2x0 F., the hardened and tempered alloy preferably exhibits a Vickers hot hardness of at least about 475 and compressive yield strength of at least about 100 ksi. The alloy preferably has a dimensional stability of less than about 0.5xc3x9710xe2x88x923 inches after 20 hours at 1200xc2x0 F.
According to a preferred embodiment, the alloy comprises a part for an internal combustion engine such as a valve seat insert for a diesel engine using EGR. The valve seat insert can be in the form of a casting or in the form of a pressed and sintered compact. Alternatively, the alloy can be a coating on the face of a valve seat insert and/or on the face of a valve seat. The alloy can also be used for wear resistant applications such as ball bearings.
According to a preferred method of making a cast alloy, the alloy is cast from a melt at a temperature of from about 2800 to 3000xc2x0 F., preferably about 2850 to 2925xc2x0 F. The alloy can be heat treated by heating to a temperature of from about 1550 to 2100xc2x0 F., quenching and tempering at a temperature of from about 1200 to 1400xc2x0 F.